Microwave duplexer



March 28, 1961 M. R. HAMILTON 2,977,552

MICROWAVE DUPLEXER Filed Sept. 15, 1958 2 Sheets-Sheet 1 N R I I E I W I L I ON MILTO FIG. 2.

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ATTORNEY March 28, 1961 M. R. HAMILTON 2,977,552

MICROWAVE DUPLEXER Filed Sept. 15, 1958 2 Sheets-Sheet 2 RECEIVER TRANSMITTER ANTENNA L2 .2 32.00 A 10 cc g g 80 2 v o 8 3, L60 1 8 0.6 c O 3 L40 O. 2 25 L20 3 0. q (I) 00 E 0 1 9325 9375 9425 2 9375 425 Frequency MC/SEC Frequency MC/SEC IF IG. 4 IF IG. 5

INVENTOR. MILTON R. HA MILTON BY dg/z" ATTORNEY.

MICROWAVE DUPLEXER Milton R. Hamilton, Beverly, Mass., assignor to Bomac' Laboratories Inc., Beverly, Mass, a corporation of Massachusetts 7 Filed Sept. 15, 1958, Ser. No. 761,224

2 Claims. (Cl. 333-13) The present invention relates to electromagnetic waveguide apparatus and more particularly to an improved crossed-guide duplexer employed in microwave radar systems.

Duplexers of the character to be described are generally employed to combine in a unitary structure the T-R and anti T-R functions necessary for operation of a single antenna microwave radar system. An example of such a device is shown in US. Patent No. 2,627,573,

issued February 3, 1953, wherein parallel waveguide structures are joined together with a coupling element disposed in the common wall formed therebetween. In the use of subject device, particularly over the range of frequencies between 9325 to 9500 megacycles; it has been noted that the amount of insertion loss at low power or during the receiving cycle resulted in a high VSWR (voltage standing wave ratio). As a result a certain amount of received signal power is not coupled into the receiver thereby reducing the usefulness of the device.

The invention has for its principal object the provision 'of an improved crossed-guide duplexer having a higher efliciency by lowering the VSWR during the receiving cycle of operation.

A further object is the provision of an improved structure in combination with the directional coupling member of a crossed-guide waveguide duplexer to thereby alter the electrical characteristics, particularly at the low power level cycle of operation.

The invention attains these objectives simply and efficiently by the provision of an electric field concentrator member within one waveguide member in proximity to the directional coupling iris. Due to the unique electric field configuration found in the crossed-guide duplexer devices, it has been determined empirically that the VSWR is considerably reduced by the placement of a probe type structure in the guide member connected to the transmitter and antenna. With the circular polarization of the field such a probe structure appears to act as a field concentrator to eliminate the prior art disadvantages.

The following detailed description will assist in an understanding of the invention together with its attendant features and advantages, reference being bad to the accompanying drawings, in which:

- Fig. l is a cross-sectional view of the embodiment of the invention;

Fig. 2 is a top plan view along the line 22 in Fig. 1;

Fig. 3 is an exploded perspective view of the illustrative embodiment partially cut away to reveal internal structure; and

Figs. 4 and 5 are performance charts.

Referring to Fig. 1 the crossed-guide duplexer 1 comprises waveguide sections 2 and 3 disposed at right angles to each other to define a common wide wall having a coupling member 4 disposed therein. The upper waveguide section 2 is hermetically sealed at the ends thereof by means of dielectric windows 5. Another window 6 isiprovided in the upper wide wall of lower waveguide section 3 below the coupling member to provide a vacu um tight chamber 7 within waveguide 2. An exhaust tubulation 8 is provided to evacuate chamber 7 and tointroduce a gaseous ionizable atmosphere of hydrogen or argon and a small amount of water vapor at reduced pressure.

Mounting flanges 9 and 10 are utilized for coupling to the components of the radar system with flange 9 connected to the receiver and flange 10 connected to a dissipative load termination. Metallic brackets 11 provide for maintaining the rectangularly disposed crossedguide relationship. To assist in the breakdown of the gas within chamber 7, a conventional ignitor electrode 12 supplies a stream of electrons directed to the coupling member 4. Additional mounting flanges 13 are disposed at the ends of waveguide 3 for coupling to the antenna and transmitter. The coupling member 4 constructed in accordance with the aforementioned patent is shown in detail in Fig. 2 and comprises a plurality of oppositely disposed extensions 20 formed by slots 21 and 22 each resonant at the middle frequency of the selected hand.

For the purpose of this disclosure it will be noted that the crossed slot coupling member 4 is disposed to one side of the longitudinal center line of each waveguide. The theory of operation of the complete duplexer arrangement is omitted herein since it is adequately discussed in the aforementioned patent.

In accordance with the present invention, improvement in the operating characteristics of the embodiment was observed after introduction of a dome-tipped metallic probe member 23 within waveguide section 3. The probe member 23 is secured to the waveguide wall opposite the common wide wall and extends into the guide a distance less than half the overall height. The center axis of the probe member was determined empirically to be displaced from the center axis of coupling member 4 to thereby lie closer to nearest narrow wall of waveguide 3 as shown in Figs. 1 and 2. Hence, the complete duplexer arrangement will be as illustrated in Fig. 3 showing the crossed-guides prior to assembly with the flanges removed for the sake of clarity. The appropriate components connected at the ends of the waveguides 2 and 3 are indicated thereon.

Results are plotted in Figs. 4 and 5 for the VSWR and insertion loss at low level fora typical duplexer operating at X-band frequencies. The solid lines 24 and 26 indicate prior art structure while the dotted lines 25 and 27 indicate the improved structure. The data shown was obtained with the probe member 23 having its center axis oriented approximately .735 inch from the antenna end of the waveguide and .180 inch from the narrow waveguide wall. The overall diameter of the probe member was .250 inch and the height to the tip of the probe from the inside wide wall was .125 inch.

There is thus disclosed an improved structure for enhancing the electrical characteristics of microwave duplexers in single antenna radar systems.

What is claimed is:

1. In a single antenna microwave frequency radio system, a duplexer comprising first and second. mutually perpendicular rectangular waveguide transmission lines having a common wide wall with a resonant directional coupling element disposed therein, said first transmission line being connected between the receiver and a dissipative load termination, said second transmission line being connected between the transmitter and the antenna,

Patented Mar. 28, 1961 a its center axis disposed" between the center axis of the coupling element and the nearest narrow wall' of said second transmission line.

2., In a microwave frequency duplexerhaving a pair of mutually. perpendicular rectangular Waveguides defining a'common wide wallwith a resonant directional coupling elementdisposed'therein, an electric" field con: centrator disposed intermediate the ends of one of said waveguides in the region of the: coupling element; said field concentrator comprising a metallic probe -member secured to the wide wall opposite the common wide wall andextending into said waveguide-toward the coupling element a distance less than half the height of said waveguide, thecenter axis'ofsaid' probe member-"beingdisplaced from the center axis of the coupling element in a direction toward the nearest narrow Wall of said 5 waveguide.

References Cited in. thefile of this patent UNITED STATES PATENTS 

